Battery-powered wristwatches utilizing liquid crystal display cells, particularly those operating by means of the field-effect, have been relatively thick due to a variety of factors. As is well knonw, minimization of the thickness of a wristwatch is highly desirable, especially in the fabrication of watches to be worn by women. The high premium placed upon watches of low thickness is such that minimization of the thickness is a major objective in the development of electronic wristwatches.
As aforenoted, a number of factors are involved in establishing the thickness of wristwatches at undesirably high levels. These factors include the necessity for a booster element in the electronic circuitry when the attempt is made to operate the liquid crystal cell and the circuitry by means of a single electrochemical cell. Conversely, when it is attempted to utilize circuitry which does not incorporate a booster circuit, then it becomes necessary to use a plurality of electrochemmical cells, namely a battery. A battery, of course, occupies space in excess of that occupied by a single cell.
In wristwatches of conventional design the battery must be large for a second reason also. Where two electrochemical cells, for instance, are required rather than one, the voltage is, of course, twice as high. As a result, the current which flows is twice as great. Consequently, the life of a battery as it operates to power a wristwatch, is cut in half. Where a single electrochemical cell is used in combination with a booster, the current which is drawn is similarly increased so that, again, the life of the electrochemical cell is cut severely. To compensate for this decrease in the life, the cell or battery size must be increased so that a greater quantity of electrochemically-active material is present. This, again, increases the size of the wristwatch. Moreover, if a booster is required, extra electronic components must be incorporated and these, in the form of coils and capacitors are space-consuming.
As is evident, a principal factor in decreasing the size of wristwatches from those now available is the nature of the liquid crystal composition used in the display cell. Where a composition can be developed which operates at a lower voltage, and consequently at a lower current, the drain is correspondingly lower and the life of the cell is increased.
One further point is significant, namely, the type of piezoelectric vibrator used for supplying a time standard. Vibrators of the conventional type have a minimum thickness of about 0.5 mm, a minimum width of about 1.5 mm, and a length of about 6 mm. The volume of such a vibrator, taking into account the space which must be provided around it, becomes substantial in terms of attempting to make a wristwatch of minimal thickness. Accordingly, it would be highly desirable to provide liquid crystal compositions which require both low voltage and low current for operation as well as a vibrator of small dimensions, the whole being operated by the power provided by a single electrochemical cell.